Two-way communication system employing two-clock frequency pseudo-noise signal modulation

ABSTRACT

A digital two-way communication system utilizing RF transmissions that are phase-shift-keyed (PSK) by binary pseudonoise (PN) code generators operating at different clock rates. Transmissions in the first and second directions are modulated by composite PN code generators operating at different first and second clock frequencies f1 and f2, respectively, where f1 NF2, with N being a positive integer having no factors in common with the lengths of the component PN coder from which the composite is formed. The binary digital data signals that are to be transmitted in either direction modulate a binary PN code which is a composite code generated from a plurality of component PN codes. Each of these data-modulated composite PN codes, which are generated at clock frequencies f1 and f2, respectively, modulates, in turn, an RF carrier signal. By having the two communication stations transmit and receive at different PN clock frequencies, equipment limitations that restrict transmitter performance in the second direction need not limit performance in the first direction, thereby allowing a higher clock frequency f1, in the first direction and subsequent lower power required at the receiver at the other end for the same SNR out of the receiver.

United States Patent Kartchner et a].

TWO-WAY COMMUNICATION SYSTEM EMPLOYING TWO-CLOCK FREQUENCY PSEUDO-NOISESIGNAL MODULATION inventors: Earl M. Kartchner, Salt Lake; Gary R. VanHorn, Granger, both of Utah; Richard A. Wallace, Phoenix, Ariz.

Assignee: Sperry Rand Corporation, New

York, N.Y.

Filed: Dec. 17, 1971 Appl. No.: 209,476

Related US. Application Data Division of Ser. No. 864,596, Oct. 8, 1969,Pat. No. 3,665,472.

US. Cl. ..235/l52, 178/22, 331/78, 340/345 Int. Cl. ..H03b 29/00, H0419/00 Field of Search 178/22; 325/39, 42; 328/158; 331/78; 340/346, 345;235/152 References Cited UNITED STATES PATENTS 2/1967 Webb .325/473/1969 Blasbalg 325/22 5/1972 Kartchner et al ..325/42 PrimaryExaminer-Benjamin A. Borchelt Assistant Examiner-H. A. BirmielAttorney-Kenneth T. Grace et al.

I 5 7 1 ABSTRACT A digital two-way communication system utilizing RFtransmissions that are phase-shift-keyed (PSK) by binary pseudo-noise(PN) code generators Operating at different clock rates. Transmissionsin the first and second directions are modulated by composite PN codegenerators operating at different first and second clock frequencies f,and 12, respectively, where f NFg, with N being a positive integerhaving no factors in common with the lengths of the component PN coderfrom which the composite is formed. The binary digital data signals thatare to be transmitted in either direction modulate a binary PN codewhich is a composite code generated from a plurality of component PNcodes. Each of these data-modulated composite PN codes, which aregenerated at clock frequencies f and f respectively, modulates, in turn,an RF carrier signal.

By having the two communication stations transmit and receive atdifferent PN clock frequencies, equipment limitations that restricttransmitter performance in the second direction need not limitperformance in the first direction, thereby allowing a higher clockfrequency f in the first direction and subsequent lower power requiredat the receiver at the other end for the same SNR out of the receiver.

2 Claims, 16 Drawing Figures XMTR PATENTEU 3 728.529

SHEEI 2 OF 6 PNI 4O CLOCK f as T' so cvc DET i can. con: I Q

K 84 JR H MAJ a a CODE X XMTR coma h 50 SYNC HWS LOGIC F-DCBS PM 42 88mum CLOCK 99 f 90 L. cvc

am 93 coo:

I CYC 5 RCVR AMPLITUDE PATENTED 3. 728.529

sum u BF 6 UOLEAN FUNCTION GENERATOR OUTPUT RELATIVE m moouwrso cmmenSIGNAL POWER. an AT necclvzn INPUT SIN CONSTANT AT OUTPUT OF RECEIVER IAH: 80 AH: 800 NH:

PN CLOCK FREQUENCY-O um um INTERFERENCE T n m D ueswse 3 mo: mo m BINTERFERENCE 2 am an 5 mnmncm 2320 2'! mi FREQUENCY FREQUE'CY SPECTRUM0F RECEIVED SIGNAL SPECTRUM OF RECEWED slaNAL FTER PN DENODULR'HON E 9Fig. n

PATENTEDAPR 1 "H975 I sum 5 or 6 mk LII? 1 EIIIIL' 33 .ICJ. .l. 2g

H XHEI PATENTEU APR 1 71873 SHEEI 6 BF 6 TWO-WAY COMMUNICATION SYSTEMEMPLOYING TWO-CLOCK FREQUENCY PSEUDO-NOISE SIGNAL MODULATIONCROSS-REFERENCE TO RELATED APPLICATION The present application is adivisional application of our U.S. Pat. application Ser. No. 864,596,filed Oct. 8, 1969, now US. Pat. No. 3,665,472.

BACKGROUND OF THE INVENTION The present invention relates tocommunication systems incorporating pseudo-noise (PN) modulation ofradio-frequency (RF) carrier signals. Such systems are well-known andhave been utilized because of their desirable spread-spectrumcharacteristics, including efficient use of signal energy, lower powertransmission and interference rejection. Such well-known systemsgenerally include at least two communicating stations of substantiallysimilar composition including clock generators that drive twopseudo-noise code generators each separately driving an associatedreceiver and transmitter which, in turn, are coupled by atransmit/receive switch or diplexer to an appropriate antenna. Thisinvention has particular reference to two-way communications involving atransponder as one of the two communicating stations.

The transmit and receive pseudo-noise code generators are generally ofthe same design, providing the desired pseudo-noise codes in a mannerwell-known in the art. The linear maximal-length or M-sequence codes areone class of PN codes. In the case of a transponder, a singlepseudo-noise code generator may drive both transmitter and receiver.Present two-way communications systems using PN modulation are designedto operate both receiver and transmitter PN code generators atessentially the same clock frequency. In the case of a remotetransponder, cost or weight restrictions may demand a transmitter thatis limited in its ability to handle wide-bandwidth, high clockfrequencyPN modulation. Generally, the wider the bandwidth of a PN-modulatedsignal the better its performance. When PN clock frequencies arerequired to be the same in both transmission directions, this limitsperformance in the baseor groundto-transponder direction to that whichis possible in the transponderto-ground direction, regardless of actualequipment capacity in the first direction.

SUMMARY OF THE INVENTION The present invention is directed toward animproved two-way communication system that employs two-clock-frequency,pseudo-noise signal modulation. Transmission between the twotransmit/receive stations, e.g., a first ground station and a secondairborne station or transponder, may employ the well-knownphaseshift-keyed (PSK) modulation of the carrier signal; see the textData Transmission," Bennett & Davey, McGraw-Hill, 1965, pp 26-31. Forpurposes of illustration, PSK will be assumed to be the technique bywhich the RF carrier is pseudo-noise (PN) modulated.

The RF carriers going in each direction may be of different frequenciesto allow simultaneous reception and transmission of signals by a commonantenna. These carrier frequencies, however, are not the ones underdiscussion here, and are not to be confused with the clock frequencies,f, and f at which the PN code generators are driven.

In the second station there is provided a composite pseudo-noise binarycode generator driven by a clock generator of frequency f,; see the text"Shift Register Sequences," S. W. Golomb, Holden-Day, l967, pp -82. Thatis, the width of an individual bit in the code is l/f,. The f, clockgenerator drives a plurality of pseudo-noise binary code generatorsproviding a like plurality of component codes of clock frequency f,which are coupled not only to a first code combiner that combines thecomponent codes into a composite code but to a second code combinerthrough associated sample-and-hold devices. The f, clock generator,through a frequency divider, divides the f, clock signal by a positiveinteger, i.e.,f,/N =f driving the sampleand-hold devices at thefrequency f whereby the set of component codes are sampled at thefrequency f providing, as output signals, composite codes of clockfrequency 1, that is, having bit widths 1/f =N/f,. The positive integerN, by which clock frequency f is divided to produce clock frequency f isrelatively prime to the cyclic lengths in bits, of the component PNcodes that are combined by some Boolean function to produce thecomposite PN code. By relatively prime is meant that the number N andthe numbers representing the lengths of the component codes have nocommon factors. The composite code of clock frequency f, from the firstcode combiner is coupled to the receiver where synchronization,acquisition and demodulation of the received signal is performed. Thecomposite code of clock frequency f, from the second code combiner iscoupled to the transmitter where modulation of the carrier signal isperformed in the well-known manner by phase-shift-keying.

The first station, in contrast, includes separate sets of component codegenerators, each set similar to the set in the second station, for thetransmitter and receiver circuits. Each set is driven at the respectivefrequencies off and f by a clock generator of frequency f, and anotherclock generator of frequency f,, i.e-,fi/N =f Separate clocks and codegenerators are necessary in the ground station so that transponder rangemay be determined by measuring the two-way RF propagation delay timeindicated by the code phase difference between the transmitter andreceiver PN code generators when the two stations are synchronized.Separate receiver and transmitter code combiners, similar to those inthe second station, generate the associated composite codes, similar tothose in the second station, directly driving the receiver andtransmitter. The composite code of frequency from the receiver codecombiner is coupled to the receiver where synchronization, acquisitionand demodulation of the received signal is performed, as in the secondstation receiver whose PN code is generated at a clock frequency f,. Thecomposite code, of clock frequency f,, from the transmitter codecombiner is coupled to the transmitter where modulation of the carriersignal is performed, as in the modulation of the carrier signal by thesecond station transmitter whose PN code is generated at a clockfrequency f,. By utilizing two different pseudonoise clocks (arelatively wide-bandwidth IN-modulated transmission from the firststation as compared to the narrower bandwidth PN-modulated transmissionfrom the second station), the power output of the first stationtransmitter may remain the same while substantially increasing thedesired anti-jam, covert, acquisition and reception performance at thereceiver of the second station.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an illustration of a blockdiagram of a prior art communication system.

FIG. 2 is an illustration of a block diagram of a communication systemincorporating the present invention.

FIG. 3 is an illustration of a block diagram of the two clock frequencyPN generator of the ground station of FIG. 2.

FIG. 4 is an illustration of a block diagram of the two-clock frequencyPN generator of the airborne station of FIG. 2.

FIGS. 5a, 5b, 5c are diagrammatic illustrations typical of the PNcomponent code generators utilized in FIGS. 3 and 4; FIG. 5d is a moregeneralized block diagram of such generators.

FIG. 6 is an illustration of a plot of PN clock frequency versusrelative PN-modulated carrier signal power required at the receiverinput to maintain a constant SNR at receiver output.

FIGS. 7a, 7b are illustrations of plots of the interference rejectingcharacteristics of the received and of the PN code-demodulated signalspectrums, respectively, versus signal amplitude.

FIG. 8 is an illustration of component codes A, B, K generated by thetwo clock-frequency PN generators of FIG. 4.

FIG. 9 is an illustration of the composite code MAJ derived from thecomponent codes of FIG. 8.

FIG. 10 is an illustration of the digital data signal data that is to bemodulated by the composite codes MAJ and MAJ.

FIG. 11 is an illustration of the PSK modulation of the carrier signalby the modulated digital data signal data-MAJ of FIG. I0.

FIG. 12 is an illustration of the PSK modulation of the carrier signalby the modulated digital data signal DATAMAJ' ofFIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With particular reference toFIG. 1 there is presented an illustration of a block diagram of a priorart communication system. Such system includes at least twocommunicating stations of substantially similar compositions, identifiedas the ground station and the airborne station. The ground stationincludes two pseudo-noise (PN) clock generators 10, ll of frequency f,that drive two associated PN code generators 12, 14 each of whichseparately drive an associated receiver 16 and transmitter 18 which, inturn, are coupled by a transmit/receive switch 20 to an appropriateantenna 22. The airborne station includes a PN clock generator 24 offrequency f, that drives a single PN code generator 26 which drives anassociated receiver 28 and transmitter 30 which, in turn, are coupled bya transmit/receive switch 32 to an appropriate antenna 34. Transmissionbetween the ground station and the airborne station is at a carriersignal ofa radio frequency (RF) that is orders of magnitude greater thanthat of the bidirectional PN clock frequencyfl.

With particular reference to FIG. 2 there is presented an illustrationof a block diagram of a communication system incorporating the presentinvention. As in the embodiment of FIG. 1, the communication systemincludes a ground station and an airborne station. However, in contrastto the embodiment of FIG. 1 where transmission between the groundstation and the airborne station is at a bidirectional PN clockfrequency of f in the embodiment of FIG. 2 transmission by the groundstation to the airborne station is at a first PN clock frequency f,while transmission by the airborne station to the ground station is at asecond PN clock frequencyf, where j" Nf, with N being a positive integerrelatively prime with respect to the component PN code lengths. The RFcarrier signal frequency is orders of magnitude greater than that of thehigher PN clock frequency f,. In addition, the RF carrier signalfrequency may be different for each transmission direction, e.g., ahigher frequency carrier signal may be PSK modulated by a PN signalhaving a high clock frequency f, while a lower frequency carrier signalmay be PSK modulated by a PN signal having a lower clock frequency f,.

The ground station includes a PN f, clock generator 40 which is coupledto PN code generator 46. A PN f clock generator 42 drives PN codegenerator 44. PN code generator 44 and PN code generator 46 eachseparately drive an associated receiver 48 and transmitter 50,respectively, which, in turn, are coupled by a transmit/receive switch,or diplexer, 52 to an appropriate antenna 54.

In the airborne station there is provided a twofrequency PN generatorcomprised of PN f clock generator 56, frequency divider S8, PN codegenerator and binary sample-and-hold device 62. PM f clock generator 56drives, in parallel, PN code generator 60 and frequency divider 58which, in turn, drives binary sample-and-hold device 62. PN codegenerator 60 drives, in parallel, receiver 64 and binary sample-andholddevice 62 which, in turn, is sampled by frequency divider 58 at afrequency f /N =f,. Binary sample-andhold device 62, in turn, drivestransmitter 66', receiver 64 and transmitter 66 are, in turn, coupled bya transmit/receive switch, or diplexer, 68 to an appropriate antenna 70.

With particular reference to FIG. 3 and FIG. 4 there are presentedillustrations of the block diagrams of the two separate PN codegenerators of the ground station and the two-cIock-frequency PN codegenerator of the airborne station, respectively, of FIG. 2. The PN codegenerators of FIG. 3 and 4 generate:

a plurality of PN component codes A, B, K of lengths L L L at a clockfrequency f, which component codes are combined in a code combiner togenerate a PN composite code X ofa length L which is a product of thelengths L L L, of the component codes and is of a block frequency f,;

a plurality of PN component codes A, B', K of lengths L,,', L L at aclock frequency f which component codes are combined in a code combinerto generate a PN composite code X ofa length L, which is the product ofthe lengths L L L of the component codes and is of a clock frequency fThe composite code X of clock frequency f is utilized:

in the ground station by the transmitter 50 for PN modulation of thetransmitted carrier signal at a clock frequencyf and in the airbornestation by the receiver 64 for synchronization, acquisition and PNdemodulation of the received signal at a clock frequencyf The compositecode X of clock frequency f, is utilized:

in the airborne station by the transmitter 66 for PN modulation of thetransmitted carrier signal at a clock frequency of f,; and in the groundstation by the receiver 48 for synchronization, acquisition and PNdemodulation of the receiver signal at a clock frequency of f,.

In the two PN code generators of FIG. 3, PN f, clock generator 40 drivesa plurality of PN component code generators 80, 82, 84. Code generators80, 82 84 generate associated PN component codes A, B, K of lengths L LL, of a clock frequency f,. Component codes A, B K are, in turn, coupledin parallel to MAJ code combiner 86 which generates a PN MAJ compositecode X of a length L, which is the product of the lengths L L L of thecomponent codes and of a clock frequency f,. Composite code X drivestransmitter 50 for PN modulation of the transmitted carrier signal at aclock frequency off Cycle detectors 81, 83, 85, which are coupled totheir associated code generators 80, 82 84, respectively, providesynchronization for data sampling and data modulation of the PN signal,coupling their associated trigger signal to synchronization logic 88which, in turn, generates appropriate word sync (WS) signals and commandbit sync (CBS) signals, which are, in turn, coupled to transmitter 50.Code select switches 89 are utilized by, and are a part of, codegenerators 80, 82 84 to generate a predetermined PN component code aswill be more fully discussed with particular reference to FIGS. 50, 5b,5c.

PN f clock generator 42 drives, in parallel, a plurality of PN componentcode generators 90, 92, 94. Code generators 90, 92 94 generateassociated PN component codes A, B, K of lengths L,,', L L, of a clockfrequencyf where f, =f,/N. N is a positive integer relatively prime toL,,, L L Component codes A, B, K, are, in turn, coupled in parallel toMAJ code combiner 96 which generates a PN MAJ composite code X of alength L,,' which is the product of component lengths L L L and is of aclock frequency f,. Composite code X drives receiver 48 for PNdemodulation of the received carrier signal at a clock frequency],

In a manner similar to that described above, cycle detectors 9,1 93, 95,which are coupled to their associated code generators 90, 92, 94,respectively, provide synchronization for data demodulation, couplingtheir associated trigger signals to synchronization logic 98 whichgenerates appropriate word sync (WS') signals and data bitsynchronization (DBS') signals, which are coupled to receiver 48. Codeselect switches 99, in a manner similar to code select switches 89 asdiscussed above, are utilized by, and are a part of, code generators 90,92 94 to generate a predetermined PN code sequence as will be more fullydiscussed with particular reference to FIGS. 50, Sb, 5c.

The two-clock frequency PN generator of FIG. 4 is an improvedarrangement of the two clock frequency PN generator of FIG. 3,performing the same functions with fewer component parts providing aless expensive, light-weight version for the airborne station of FIG. 2.In the two-clock frequency PN generator of FIG. 4, PN

5 f clock generator 56 drives, in parallel, a plurality of PM componentcode generators 100, 102, 104 and frequency divider 58. Code generators100, 102, 104 generate associated PN component codes A, B, K of lengthsL L L, at a clock frequency f, in a manner similar to that discussedwith particular reference to FIG. 3. Each of the component codes A, B, Kare, in turn, coupled in parallel to MAJ code combiner 106 andassociated binary sample-andhold devices 110, 112, and 114,respectively. Binary sampleand-hold devices 110, 112, 114 sample, at afrequency f,, the respectively associated component codes A, B, K whichwere generated at a clock frequency f, and generate, at a clockfrequency f,, respectively associated PN component codes A, B, K,oflengths L L L wheref =f,/N. Component codes A, B, K are, in turn,coupled in parallel to MAJ code combiner 108 which generates a PN MAJcomposite code X of a length L L, X L, X L, at a clock frequency f,.Composite code X drives transmitter 66 for PN modulation of thetransmitted carrier signal at a clock frequency f In a manner similar tothat of the above discussed arrangement of FIG. 3, cycle detectors 101,103, 105, which are coupled to their associated code generators 100,102, 104, respectively, provide synchronization for data sampling anddata modulation and demodulation of the PN signals X and X, couplingtheir associated trigger signal to synchronization logic 116 whichgenerates appropriate word sync (WS) signals, and also data bitsynchronization (DBS) signals and command bit synchronization (CBS)signals, which are, in turn, coupled to transmitter 66 and receiver 64,respectively. Code switches l 18 are utilized by, and are a part of,code generators 100, 102, 104 to generate a predetermined PN code aswill be more fully discussed with particular reference to FIGS. 5a, 5b,5c.

Prior to discussing, in detail, the PN component code generators ofFIGS. 50, 5b, 5c, reference will be made back to the preferredembodiment of the present invention as illustrated in FIG. 2. Byutilizing a two-clockfrequency pseudo-noise modulated transmission,(e.g., a different clock frequency for each of the two directions ofcommunication) there are provided many advantages over prior artarrangements such as illustrated in FIG. 1. One advantage of a two-waycommunication system utilizing PN modulation at different clockfrequencies in either of the two directions of transmission, is thepossibility of trading-off transmission carrier signal power for PNclock frequency. With reference to FIG. 6, there is presented anillustration of a plot of PN clock frequency versus PN-modulated carriersignal power. FIG. 6 illustrates that for a given signal-to-noise ratioout of the receiver, i.e., S/N constant, the PN clock frequency may beincreased while decreasing the PN-modulated carrier signal power intothe receiver. Thus, by transmitting from the ground station to theairborne station at a higher PN clock frequency f,, and by transmittingfrom the airborne station to the ground station at a lower clockfrequency f the same receiver output signal-to-noise ratios may bemaintained while substantially decreasing the power requirements of theground transmitter as compared to those of the airborne transmitter.

Additionally, by utilizing the two clock-frequency PN generators ofFIGS. 3 and 4 there is provided a twoway spread-spectrum communicationsystem having an improved resistance to narrow band interference in onedirection even though its performance is restricted by equipmentlimitations in the other directions. With particular reference to FIGS7a, 7b there are presented illustrations of plots of the interferencecharacteristics of the received signal and of the PN demodulated signalspectrums, respectively, versus transmitted signal amplitude. Modulationof the transmitted carrier signal by a binary sequence, such as the MAJcode sequence X, is a form of scrambling that spreads the transmittedsignal spectrum such as illustrated in FIG. 70. By demodulating thereceived signal by the known MAJ code sequence X the received messagespectrum is collapsed while, at the same time, any CW interference thatis present is spread, while uncorrelated broadband noise is notcollapsed. The received signal may then be separated by a narrow bandfilter. Such spread-spectrum characteristics of the received signal areillustrated in FIG. 7b. For a further discussion of such techniques seethe publication An Introduction To Pseudo Noise Modulation" .I. P.Chandler, AD 479308.

With particular reference to FIGS. 50, b, 5c there are presenteddiagrammatic illustrations of typical PN component code generators thatcould be utilized in FIGS. 3 and 4; a more generalized block diagram isshown in FIG. 5d. Utilizing the two-clock frequency PN generator of FIG.4 as an illustrative example, the PN code generators of FIG. 5a, 5b, 50may be considered to be analogous to PN code generators 100, 102 and104. Such PN code generators consist essentially of a basic shiftregister to which modulo-two adders have been added. These modulo-twoadders, which perform the exclusive-OR logic function, are insertedbetween adjacent stages of the shift register while the outputs from thestages from the selective inputs to the modulotwo adders so that singleor multiple closed feedback loops are formed thereby. When the shiftregister is clocked in the normal manner, the output from any stage ofthe shift register (normally the right hand stage) forms a digital PNcoded sequence. In the general case, the ensuing digital coded sequencedepends on both the feedback connections and on the initial loading (orcontent) of the shift register with the ensuring digital coded sequencebeing generated at a frequency established by the shift register clocksignal 120, which in the embodiment of FIG. 4 is a PN clock of frequency1']. For a thorough discussion of the theory of operation of such PNcode generators see the publi cation Introduction To Linear ShiftRegister Generated Sequences" T. G. Birdsall et 211., Ad 225380.

With particular reference to Tables A, B, C, there are illustrated thecontents of the respectively associated shift registers of FIGS. 50, 5b,5c, respectively, at the respectively associated bit times. As noted inTables A, B, C, the shift register stages of the PN component codegenerators of FIGS. 50, 5b, 50 have all their stages I through ninitially loaded with all ls" whereby at successive PN bit times Ithrough (2" -l) the last, or nth stage is caused to emit a linearmaximallength (M) sequence as is well-known in the art; see thepublication Study of Linear Sequence Generators," C. C. Hoopes et al.,AD 4887 I 8.

TABLE A STAGE I 2 3 B l I I I I2 I I 0 T 3 O I I 4 I 0 O T 5 0 I 0 I6 00 I M 7 I 0 I E I I 1 I TABLE 8 STAGE I 2 3 4 B I I I I I I 2 I 0 I I T3 I 0 0 I 4 I 0 0 0 5 O I 0 0 T 6 0 0 I 0 I7 0 O 0 I M 8 l I 0 0 E 9 0 II 0 I0 0 0 I I II I I 0 1 I2 I O I 0 I3 0 I O l 14 I I I 0 I5 0 I l I II I I I TABLE C STAGE I 2 3 4 5 B I I I I I I I 2 I I 0 l I T 3 I I 0 OI 4 I I 0 O O 5 0 I I O O 6 0 0 I I 0 7 0 0 O I I T 8 I 0 I 0 I I 9 I II 1 0 M 10 0 I I I I E II I 0 0 I I 12 l I I 0 I l l 0 I 0 I4 0 I I 0 II5 I 0 0 I 0 l6 0 I 0 0 I I7 I 0 0 0 0 I8 0 I O 0 0 I9 0 0 I 0 0 10 0 00 I 0 2| 0 0 O 0 l 22 I O I U 0 23 0 I 0 I O 24 O 0 I (1 25 I 0 I I O 260 l 0 I I 27 I 0 0 I) I 28 I I I 0 0 29 0 I I I 0 30 O 0 I I I 31 l O ll l I I I I I I TABLE D MAJ=AB+BK+AK A B K MA] 0 0 0 0 0 0 I 0 0 I 0 0 0I I I I 0 0 0 The interstage modulo-two adders, represented by thesymbolEB, form inputs from the feedback path, from the last stage n tothe first stage I, as determined by the respectively associated codeselect switches SI through Sn-I; switches 81 Sn-l of FIGS. 5a, 5b, 5care represented in e.g., FIG. 4 by code select switches 118 anddetermine the PN component codes, or, in this example, M-sequences, thatare generated by component code generators 100, 102, 104. As an example,with component code generator 100 of FIG. 5a having switch S1 opened andswitch S2 closed and with an initial content of all Is" in stages 1, 2,3, successive clock pulses 120 at PN bit times t, --t, cause componentcode generator I to generate and emit from its last stage, n 3, the Msequence lOlOOll of 7 bits in length which M-sequence is cyclicallyemitted therefrom as indicated by Table A. Likewise, the illustratedopened, or closed, status of switches SI Sn l of component codegenerators I02, I04 as noted in FIGS. 5b, 5c, respectively, causescomponent code generators 102, 104 to generate and emit from their laststages, n 4, n 5, respectively, the M sequences and IIIOOOIOOI lOlOl ofbits in length, 1110001 l0lll0l0l0000l00l0l IOOII of 3t bits in length,respectively. With particular reference to FIG. 8 there M-sequencecomponent codes, as generated by component code generators I00, 102. aI04, respectively, are noted as component codes A, B, K wherein the highlevel signal represents a l and the low level signal represents a 0".

FIG. 8 illustrates, at a PN bit time base where one sample-and-holdpulse occurs at a frequency f; =f lN, (N l l an initial small portion ofthe cyclical sequence, for FIG. 4, of component codes A, B K which arethe inputs to MAJ code combiner I06 and the modulation, or division,thereof by the associated binary sample-and-hold devices 110, 112, 114generating the component codes A, B, K, respectively, which are theinputs to MAJ code combiner 108. FIG. 8 likewise illustrates thecyclical sequence, for FIG. 3, of component codes A, B, K generated bycomponent code generators 80, 82, 84, respectively, which are the inputsto MAJ code combiner 86 and of component codes A, B, K, generated bycomponent code generators 90, 92, 94, respectively, which are the inputsto MAJ code combiner 96.

With particular reference to FIG. 9 there are illustrated, at the samePN bit time base as FIG. 8, the binary signal wave forms of the outputsof the code combiners that are associated with FIGS. 3 and 4. Table Dpresents the truth table of the logical operation performed by the codecombiners utilized in FIGS. 3 and 4.

With particular reference to FIG. 10 there is presented an illustration,where I data bit time equals PN bit times, of the digital data signalthat is to be transmitted and the modulation thereof by the MAJ and MAJcomposite codes of FIG. 9. FIG. [0, using the same time base as that ofFIGS. 8, 9, illustrates that the MAJ (MAJ') composite code is modulatedby the digital data signal; if of a high level, representative of al,"it provides a true output of the digital data signal while if of alow level, representative of a 0", it provides the complement of thedigital data signal.

With particular reference to FIG. I! there is presented an illustration,at the same time base as FIGS. 8, 9, 10, of the PSK modulation of thecarrier signal by the modulated digital data signal DATA- MAJ' of FIG.10. The RF carrier signal of FIG. 11 is illustrated as being on the sametime base as FIG. 10, being at a frequency j" of approximately f,/2, forthe purpose of illustrating that a plurality of carrier signal cyclesoccurs during each DATA-MAJ bit time period, the five cycles illustratedin FIG. I 1 being for illustrative purposes only, no limitation theretointended.

With particular reference to FIG. 12 there is presented an illustration,with the time base being 10 times that of FIGS. 8, 9, 10, ll, of PSKmodulation of the carrier signal by the modulated digital data signalDATA-MAJ of FIG. 10. The RF carrier signal of FIG. 12 is illustrated asbeing on a different time base than FIG. 11, being at a frequency f ofapproximatelyf,/l0, for the purpose of illustrating that, as with FIG.11, a plurality of carrier signal cycles occurs during each DATA-MAJ bittime period, the number of cycles illustrated in FIG. 12 being forillustrative purposes only, no limitation thereto intended.

It is to be understood that the signals of FIGS. 11, 12 are presentedfor illustrative purposes only, PSK modulation being only one of severalways in which the RF carrier may be modulated by a PN sequence. Theimplementation is performed by the receivers of FIGS. 3 and 4 for thedemodulation of the received signal and by the transmitters of FIGS. 3and 4 for the modulation of the carrier signal, such procedures beingperformed in well-known manners.

Thus, it is apparent that applicants have presented a noveltwo-clock-frequency PN generator providing an improved digital datatwo-way communication system utilizing different clock-frequencypseudo-noise modulated RF transmission signals.

What is claimed is:

l. A two clock-frequency PN generator, comprising:

a plurality of A, B K of PN component code generators for generating anassociated plurality of A, B, K PN component codes of associateddifferent lengths L L L a first clock generator for generating a firstclock signal of a frequencyf a second clock generator for generating asecond clock signal of a frequency f,, where f, Nf with N being apositive integer relatively prime to code lengths L,,, L L

means for coupling said first clock signal of frequencyf to saidplurality of A, B, K PN component code generators for generating saidplurality of A, B, K PN component codes at said PN bit frequencyfl;

a sampling device;

means coupling said plurality of A, B, K PN component codes to saidsampling device, and

means coupling said second clock signal of frequency f: to said samplingdevice for causing said sampling device to generate a plurality of PNcomponent codes A, B, K of associated different lengths LA', LB, Ln;

first code combiner means;

means coupling said plurality of A, B, K PN component codes to saidfirst code combiner means for generating a PN composite code X of a PNbit frequency f and of a length L,,' (L,,') (L,,') (L the product of thecomponent code lengths;

second code combiner means;

means coupling said plurality of A, B, K PN component codes to saidsecond code combiner means for generating a second PN composite code Xof a PN bit freqnencyf and ofa length L,, (L,,) (L n);

2. A two clock-frequency PN generator, comprising:

a plurality of A, B, K of PN component code generators for generating anassociated plurality of A, B, K PN component codes of associateddifferent lengths L L L a first clock generator for generating a firstclock signal ofa frequency f,;

a second clock generator for generating a second clock signal of afrequency jg, where f, Nf with N being a positive integer relativelyprime to code lengths L L L means for coupling said first clock signalof frequencyf to said plurality of A, B, K PN component code generatorsfor generating said plurality of A, B, K PN component codes at said PNbit frequencyfl;

a sampling device;

means coupling said plurality of A, B, K PN component codes to saidsampling device, and

means coupling said second clock signal of frequency f to said samplingdevice for causing said sampling device to generate a plurality of A, B,K PN component codes of associated different lengths L3, L LA";

MA! code combiner means;

means coupling said plurality of A, B, K PN component codes to said MAJcode combiner means for generating corresponding MAJ PN composite codeof a PN bit frequency f, and of length (L (L (L the product of thelengths of the component codes",

MAJ code combiner means;

means coupling said plurality of A, B, K PN component codes to said MAJcode combiner means for generating corresponding MAJ PN composite codesof :1 PN bit frequencyf and of a length (L (L,,) (L,,), the product ofthe lengths of the component codes.

1. A two clock-frequency PN generator, comprising: a plurality of A,B - - K of PN component code generators for generating an associatedplurality of A, B, - - K PN component codes of associated differentlengths LA, LB, - - LK; a first clock generator for generating a firstclock signal of a frequency f1; a second clock generator for generatinga second clock signal of a frequency f2, where f1 Nf2 with N being apositive integer relatively prime to code lengths LA, LB, - - - LK;means for coupling said first clock signal of frequency f1 to saidplurality of A, B, - - K PN component code generators for generatingsaid plurality of A, B, - - K PN component codes at said PN bitfrequency f1; a sampling device; means coupling said plurality of A,B, - - K PN component codes to said sampling device, and means couplingsaid second clock signal of frequency f2 to said sampling device forcausing said sampling device to generate a plurality of PN componentcodes A'', B'', - - K'' of associated different lengths LA , LB , - - LK; first code combiner means; means coupling said plurality of A'',B'', - - K'' PN component codes to said first code combiner means forgenerating a PN composite code X'' of a PN bit frequency f2 and of alength LX'' (LA'') (LB'') - - (LK''), the product of the component codelengths; second code combiner means; means coupling said plurality of A,B, - - K PN component codes to said second code combiner means forgenerating a second PN composite code X of a PN bit frequency f1 and ofa length LX (LA) (LB) - - (LK);
 2. A two clock-frequency PN generator,comprising: a plurality of A, B, - - K of PN component code generatorsfor generating an associated plurality of A, B, - - K PN component codesof associated different lengths LA, LB - - LK; a first clock generatorfor generating a first clock signal of a frequency f1; a second clockgenerator for generating a second clock signal of a frequency f2, wheref1 Nf2 with N being a positive integer relatively prime to code lengthsLA, LB, - - LK; means for coupling said first clock signal of frequencyf1 to said plurality of A, B, - - K PN component code generators forgenerating said plurality of A, B, - - K PN component codes at said PNbit frequency f1; a sampling device; means coupling said plurality of A,B, - - K PN component codes to said sampling device, and means couplingsaid second clock signal of frequency f2 to said sampling device forcausing said sampling device to generate a plurality of A'', B'', - -K'' PN component codes of associated different lengths LA , LB , - - LK; MAJ'' code combiner means; means coupling said plurality of A'',B'', - - K'' PN component codes to said MAJ'' code combiner means forgenerating corresponding MAJ'' PN composite code of a PN bit frequencyf2 AND of length (LA'') (LB'') - - (LK''), the product of the lengths ofthe component codes; MAJ code combiner means; means coupling saidplurality of A, B, - - K PN component codes to said MAJ code combinermeans for generating corresponding MAJ PN composite codes of a PN bitfrequency f1 and of a length (LA) (LB) - - (LK), the product of thelengths of the component codes.